Rates of Ligand Exchange between >FeIII-OH2 Functional Groups on a Nanometer-Sized Aqueous Cluster and Bulk Solution
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- 作者：Edina Balogh ; Ana Maria Todea ; Achim Mü ; ller ; William H. Casey
- 刊名：Inorganic Chemistry
- 出版年：2007
- 出版时间：August 20, 2007
- 年：2007
- 卷：46
- 期：17
- 页码：7087 - 7092
- 全文大小：213K
- 年卷期：v.46,no.17(August 20, 2007)
- ISSN：1520-510X
文摘
Variable-temperature 17O NMR experiments were conducted on the nanometer-sized Keplerate Mo72Fe30 cluster,with the stoichiometry [Mo72Fe30O252(CH3COO)12[Mo2O7(H2O)]2[H2Mo2O8(H2O)](H2O)91]·~150H2O. This moleculecontains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the ratesof exchange of the isolated >FeIII-OH2 waters with bulk aqueous solution. Both longitudinal and transverse 17O-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: kex298 = 6.7(±0.8) × 106s-1, which is about a factor of ~4 × 104 times larger than the corresponding rate coefficient for the Fe(OH2)63+ ionof kex298 = 1.6 × 102 s-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and 
hars/Delta.gif" BORDER=0 >H
hermod.gif"> and hars/Delta.gif" BORDER=0 >Shermod.gif"> are 26.3 ± 0.6 kJmol-1 and -26 ± 0.9 J mol-1 K-1, respectively. High-pressure 17O NMR experiments were also conducted, but thecluster decomposed slightly under pressure, which precluded confident quantitative estimation of the hars/Delta.gif" BORDER=0 >Vhermod.gif">. However,the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as aD or Id mechanism. The enhanced reactivity of waters on the Mo72Fe30 cluster is associated with an increase inthe FeIII-OH2 bond length in the solid state of ~0.1 Å relative to the Fe(OH2)63+ ion, suggesting that a correlationexists between the FeIII-OH2 bond length and kex298. Although there are only few high-spin Fe(III) complexeswhere both exchange rates and structural data are available, these few seem to support a general correlation.
hars/Delta.gif" BORDER=0 >Hhermod.gif"> and hars/Delta.gif" BORDER=0 >Shermod.gif"> are 26.3 ± 0.6 kJmol-1 and -26 ± 0.9 J mol-1 K-1, respectively. High-pressure 17O NMR experiments were also conducted, but thecluster decomposed slightly under pressure, which precluded confident quantitative estimation of the hars/Delta.gif" BORDER=0 >Vhermod.gif">. However,the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as aD or Id mechanism. The enhanced reactivity of waters on the Mo72Fe30 cluster is associated with an increase inthe FeIII-OH2 bond length in the solid state of ~0.1 Å relative to the Fe(OH2)63+ ion, suggesting that a correlationexists between the FeIII-OH2 bond length and kex298. Although there are only few high-spin Fe(III) complexeswhere both exchange rates and structural data are available, these few seem to support a general correlation.